Seeing the Light


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Prof. Mordechai Sheves. microsecond shift
Scientists are just beginning to understand how plants use light for nourishment and people use light to see. Presently a team of researchers, among them Weizmann Institute scientists, have put into question what happens in the first millionth of a millionth of a second after light strikes a plant cell or our retina.
Until now it was thought that that the rotation of a molecule called retinal initiated a cascade of events leading to vision. However, Prof. Mordechai Sheves of the Weizmann Institute's Organic Chemistry Department, who succeeded in "locking" retinal so that it couldn't rotate, showed that the initial process of vision was not thereby prevented.
Both in animals and in plants, photons (particles of light) are converted into electric energy upon striking certain proteins and changing their structure. This complex process is not quite understood. The protein that absorbs light in plants is a green pigment called chlorophyll. In the human eye the proteins absorbing light are blue, green, and red pigments. At night, black and white images are formed using a single pigment called rhodopsin. The scientists studied the rhodospin protein, a version of which can be found in certain bacteria.
Retinal, a small molecule that is present in all retinal proteins, including rhodospin, was thought to be responsible for the first stage in the process of vision. A light beam striking retinal causes it to change structurally and to turn. This is one of the swiftest processes known in biochemistry, taking place in a minuscule period of time -- less than a tenth of a millionth of a millionth of a second. Until now the rotation of retinal, which is located in a small pocket found inside the proteins, was believed to trigger a structural change in the entire protein, causing the conversion of a photon to electric energy. Now scientists have proven that the process begins even before the rotation of retinal.
Sheves, collaborating with Profs. Michael Ottolenghi, Sandy Ruhman, and Aharon Lewis of the Hebrew University, exposed proteins containing the "locked" retinals (unable to turn) to very brief light flashes -- one-tenth of a millionth of a millionth of a second. The scientists were surprised to discover that a large number of the processes that occur in the natural system take place in the "locked" system as well. This finding has led to the conclusion that the rotation of retinal as a result of light detection is not the first stage in the sequence of processes that result in vision.

The retinal molecule. shape shifter

Using an atomic force microscope (AFM), the researchers discovered that the structure of the entire protein changed as a result of light absorption even when retinal was "locked." Sheves: "We began to monitor the changes in the retinal protein mere microseconds after the light had been detected by it, and we noticed that on detecting the light ? the protein changes its structure."

What, then, intiates the change in the protein? Researchers surmise that as a result of absorbing the light, retinal undergoes a change expressed in the dispersal of an electric charge along the molecule. This rapid change may induce additional changes in the charges of the proteins that surround retinal, and these cause a change in the structure of the entire protein. This, then, is -- probably -- the first stage of the process, and it occurs in less than a millionth of a millionth of a second, even before retinal turns.

Center of Excellence

Prof. Mordechai Sheves of the Weizmann Institute is collaborating with Prof. Michael Ottolenghi, Prof. Sandy Ruhman, and Prof. Aharon Lewis of the Hebrew University of Jerusalem to uncover the molecular processes at the root of energy conversion in bacteriorhodopsin. The team has been declared a Center of Excellence of the National Science Foundation.